In a switching circuit that is a main element of an inverter unit in a motor control device, a switching element, arranged between opposite-polarity DC busbars, performs switching of a bus voltage to generate driving power to a motor. The bus voltage of this switching circuit is a DC voltage that is stabilized by a smoothing capacitor by removing a ripple component from a DC voltage obtained by rectifying an input AC voltage by a converter circuit. The smoothing capacitor that forms and stabilizes a bus voltage of the switching circuit as described above is referred to as “main-circuit capacitor”.
In a motor control device, a ripple current caused by a ripple component included in an output current from a converter circuit flows to a smoothing capacitor. A ripple current caused by a ripple component included in regenerative power that is output from a switching circuit to DC busbars at the time of a regenerative operation of a motor driven by an inverter unit also flows to this smoothing capacitor.
When a ripple current flows, the smoothing capacitor that is constituted by an electrolytic capacitor generates heat. Due to thermal stress associated with a temperature increase in an aluminum electrolytic capacitor used as the smoothing capacitor, the life of the aluminum electrolytic capacitor is shortened according to the Arrhenius law.
However, in a motor control device including a switching circuit that switches ON/OFF a switching element to generate driving AC power to a motor, because of the structural reasons of the motor control device, it is difficult to arrange a current sensor that detects a ripple current that flows to a smoothing capacitor. Therefore, in a conventional motor control device, a method of diagnosing shortening of the life of a smoothing capacitor without using a ripple current has been adopted. Three specific examples are described below.
First conventional example: A method has been known, in which upon switching OFF a main power supply of a motor control device, a constant current I flows for a given period Δτ to measure a voltage drop ΔV in a smoothing capacitor during the given period Δτ, and then a residual capacitance C of the smoothing capacitor is calculated using the equation “C=(I×Δτ)/ΔV”, and is compared with an allowable value of the capacitor capacitance, thereby diagnosing shortening of the life offline. In this first conventional example, the residual capacitance of the smoothing capacitor can be calculated with high accuracy without calculating the amount of thermal stress.
Second conventional example: A technique has been proposed in Patent Literature 1, in which there are provided a unit that detects the temperature of a smoothing capacitor, a unit that calculates an amount of thermal stress in the smoothing capacitor by using a coefficient normalized by a carrier frequency and a coefficient normalized by an output voltage command, and a unit that accumulates the amount of thermal stress during an operating time of a motor control device, in which output power to a motor is calculated from the voltage command, and the amount of thermal stress in the smoothing capacitor is calculated from the output power, thereby computing the accumulated life of the smoothing capacitor. In this second conventional example, by accumulating the amount of thermal stress in the smoothing capacitor, shortening of the life of the smoothing capacitor can be predicted online.
Third conventional example: A technique has been proposed in Patent Literature 2, in which based on a prediction result of shortening of the life of a smoothing capacitor, at the point in time when the smoothing capacitor is predicted to have reached the end of its life, output power from a motor control device is reduced or is shut-off. The motor control device disclosed in Patent Literature 2 has a configuration of determining whether the smoothing capacitor has reached the end of its life. Therefore, the smoothing capacitor is prevented from overheating, and thus from being broken. An unstable motor control caused by an increase in the amplitude of a ripple voltage associated with a decrease in the capacitor capacitance can be prevented. Further, a switching element is prevented from being applied with an overvoltage, and thus from being broken. Accordingly, the motor control device can take advantage of the endurance of the device as much as possible.